Re: [PATCH 1/2] block: accumulate segment page gaps per bio

[Keith Busch <kbusch@kernel.org>]

On Mon, Aug 11, 2025 at 06:17:56PM +0200, Christoph Hellwig wrote:
> On Mon, Aug 11, 2025 at 09:27:18AM -0600, Keith Busch wrote:
> > I initially tried to copy the nsegs usage in the request, but there are
> > multiple places (iomap, xfs, and btrfs) that split to hardware limits
> > without a request, so I'm not sure where the result is supposed to go to
> > be referenced later. Or do those all call the same split function later
> > in the generic block layer, in which case it shouldn't matter if the
> > upper layers already called it?
> 
> Yes, we'll always end up calling into __bio_split_to_limits in blk-mq,
> no matter if someone split before.  The upper layer splits are only
> for zone append users that can't later be split, but
> __bio_split_to_limits is stilled called on them to count the segments
> and to assert that they don't need splitting.

Zone write plugging presents a problem. For the same reason that
"__bi_nr_segments" exists, I have to stash this result somewhere in the
bio struct. I mentioned earlier I just need one byte, and there's a byte
hole in the bio already, so won't need to increase the size.